Cutting inserts disclosed in, for example, International Publication No. 2013/029072 (Patent Document 1) and International Publication No. 2004/080633 (Patent Document 2) have conventionally been known as a cutting insert for use in a cutting process of a workpiece. The cutting inserts disclosed in Patent Documents 1 and 2 are used in a milling process, such as a face milling process or an end milling process.
The cutting inserts respectively disclosed in Patent Documents 1 and 2 are provided with a flat cutting edge being parallel to a surface of the workpiece in order that a machining surface, namely, a surface of the workpiece to be machined by a cutting process is made into a smooth finished surface. Although the cutting insert disclosed in Patent Document 1 has the flat cutting edge, a cutting edge angle of a major cutting edge is large. The cutting edge angle is an inclination angle of the cutting edge of the cutting insert being attached to a holder with respect to the machining surface of the workpiece. That is, with the cutting insert disclosed in Patent Document 1, chips to be generated during cutting of the workpiece have a large thickness, and the major cutting edge is subjected to a large cutting resistance. Consequently, heat is apt to stay in the major cutting edge, and then the major cutting edge may fracture. It is therefore necessary to decrease a feed per revolution that is an amount of movement of an edge of a cutting tool per revolution. It is possible to decrease the cutting edge angle and increase the feed per revolution by making the flat cutting edge and the major cutting edge into a continuous straight line shape as in the cutting insert disclosed in Patent Document 2.
FIG. 1 shows a situation where the milling process is carried out using the cutting insert disclosed in Patent Documents 1 and 2. As shown in FIG. 1, the workpiece W is to be machined by rotating a plurality of the cutting inserts 1′ along an outer periphery of the holder 101′ having the plurality of the cutting inserts 1′ attached thereto, while moving the holder 101′ in one direction. During the machining, a rotation center axis Y1 of the holder 101′ is inclined at an angle θ toward an advance direction L of the holder 101′ with respect to a vertical axis Y2 relative to the machining surface of the workpiece W. In order to obtain a satisfactorily machined surface, a front edge A cuts the workpiece W with the holder 101′ inclined at the angle θ during the machining of the workpiece W, and a rear edge B needs to be moved away from a finished surface. However, the holder 101′ is subjected to a thrust force P applied vertically from the machining surface to the front edge A. The thrust force P exerted on the front edge A can cause the front edge A to move up, and the rear edge B can move down by reaction thereof. Hence, when the thrust force P is large, there is a risk that the rear edge B significantly moves down and contacts with the finished surface.
After the machining surface is machined into a smooth finished surface with less unevenness by using the flat cutting edge, the occurrence of damage to the finished surface by the rear edge B becomes a cause of poor machined products. Particularly, in the cutting insert disclosed in Patent Document 2, there remains a risk that the finished surface is damaged because the cutting edge angle of the major cutting edge being continuous with the flat cutting edge is small and the thrust force is large.
The present invention has been made in view of the above problems, and it is an object of the present invention to provide a cutting insert, a cutting tool, and a method of manufacturing a machined product, all of which make it possible to suppress damage to the finished surface while reducing the risk of fractures in the cutting edge.